# Why does the spiraling slipstream from a propeller hit the left side of the fin?

In the picture below, if the fuselage was shorter/longer, would the spiraling slipstream from a propeller hit the fin on the right side as opposed to the left?

Is it a deliberate positioning if yes, to hit the left (port) side?

I know the diagram is cartoonish but I couldn't find wind tunnel or CFD illustration.

It may seem hard to hit the right side, but imagine hitting the top of the elevator on the right side. Would this slipstream affect the pitch as well as the yaw?

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Is the following image related? I mean the path of the vortex, not the cause of the vortex.

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• I have wondered this exact same thing. I would assume the effective pitch of the spiral would elongate and shorten with airspeed relative to the plane, so yeah, why not hit the right side of the vertical stabilizer as it comes over the top of the plane just ahead of the empennage? It could strike the top of the right horizontal stabilizer and create a high pressure area; or so I would think. Jun 25, 2016 at 4:56
• Look at your diagram: the air flow always comes up the left side of the fuselage and down the right side. Therefore it will always hit the left side of the vertical stabilizer. If the prop rotated anticlockwise then it would be the opposite. Jun 25, 2016 at 11:26

## 1 Answer

The air flow is not a spiraling tube like shown on the picture for simplification, or as suggested by the condensation on the second image. Actually all air behind the propeller is:

• In rotation, the direction being impulsed by the propeller rotation. This rotation is maximal for air close to the propeller, and by effect of viscosity, a gradient exists in air until a certain distance where it is negligible.

• In translation, due to the aircraft moving forward (or air moving backward when the aircraft is holding on the ground).

The resulting motion relative to the aircraft is a cylinder of air moving in an oblique direction, as represented in your image. Note this is not a spiral with non-rotating air in between, but really a continuous cylinder.

Actually the speed/pressure is not uniform within the cylinder, there is indeed a sinusoidal undulation, but the intensity never decreases to zero.

If the fuselage was shorter/longer, would the spiraling slipstream from a propeller hit the fin on the right side as opposed to the left?

No, the result is that whatever the propeller pitch or the fin distance to the propeller, air is continuously hitting the vertical surfaces in a circular motion from the left (when seen from a location behind the aircraft).

It may seem hard to hit the right side, but imagine hitting the top of the elevator on the right side.

The right side of the fin (as well as all surfaces in the shadow of the rotating air) sees a pressure decrease.

The more the distance from the the aerodynamic center of the aircraft, the higher the torque created on the aircraft, and the higher the tendency to rotate.

Does this slipstream affect the pitch as well as the yaw?

The tendency is:

• To yaw for the vertical tail components and the fuselage.

• To bank for the horizontal tail components. This is not a pitch moment, because each side of the tail receives an opposite force. This force is balanced by the torque created by the propeller rotation on the aircraft.

• To bank for the wing. This force is also balanced by the propeller torque.

Note that because there is a pressure undulation, the surfaces are hit by a slipstream varying slightly at the frequency of the blades.

For more details and theory: Propeller-Rudder interaction (in water, also applicable to air).

Is the following image related?

The condensation happens at the tip and trailing edge of the propeller blades, and is then carried aft by the movement of the air cylinder. So it is completely related. Note that:

• Air between condensation trails moves at the same speed (except the undulation).
• There are two interleaved helices for a two-blade propeller.

Related effects: